Its not about programming languages at all. Once you know what you are doing, it shouldnt take you more than about 2 weeks to be proficient in any language. WRT object oriented programming (and even procedural programming to some extent), the basic principles remain the same: loops, if statements etc. Only the syntax changes really. If you are using an IDE like visual studio with intellisense, it makes the transition between languages a lot easier. If i were to choose a language it would c++, java, or c#. Its strange, but the syntax of java and c# is practically identical. I bet you could copy java code (excluding dependencies etc) into a c# compiler and it would just about run.
Development is about problem solving, and not only about knowing the ins and outs of a particular language.